1. Field of the Invention
The present invention relates generally to electron beam exposure apparatus and, more particularly, to an electron beam exposure apparatus for scanning electron microscopy (SEM).
2. Description of the Related Art
In conventional electron beam exposure apparatus, electron beams emitted from electron guns provided for exposure have been used as light sources for scanning in SEM. The SEM is used to make optical adjustments such as focusing the exposure lens and aligning the optical axis of an electron beam with the optical axis of the exposure lens before exposure, and such functions are interrupted when exposure is made by the electron beam.
FIG. 1 shows such an electron beam exposure apparatus. The apparatus consists of a control system for controlling respective units housed in a column 109 and a display 102 for checking SEM images.
The column 109 includes an electron gun 110, a condenser lens 111, a deflection coil 113 for deflecting an electron beam from the electron gun 110 so that the electron beam draws a predetermined pattern on a wafer 117 (or mask) put on a stage 116, an objective lens 114, and a secondary electron detector 115 for detecting secondary electrons generated by irradiation of the electron beam from the electron gun 110 onto the wafer 117. A pump 118 is provided to degas the column 109.
The control system includes a control computer 101, a column control 103, a vacuum control 104, a stage control 105, and an image memory 107. All instructions for the respective controls are issued by the control computer 101.
In exposure operation, the column control 103 controls the emission of electron beams from the electron gun 110 for exposure, the deflection by the deflection coil 113, and the focusing of the condenser lens 111 and the objective lens 114 so as to control the exposure operation.
The vacuum control 104 drives the pump 118 to keep the column 109 at a constant vacuum. The stage control 105 moves the stage 116 (X, Y directions and rotation) to a predetermined position at exposure and SEM recording.
In SEM operation, the column control 103 sends scanning control signals similar to those of the exposure operation to the respective units within the column for making scanning control. Also, the scanning control signal is sent to the image recording unit 107 wherein a signal indicative of a scanned image is generated from the output of the secondary electron detector 115 at respective scanning positions indicated by the scanning control signal and sent to the display 102 for showing the scanned image.
FIG. 2 shows the column control unit 103 and the image recording unit 107. It shows mainly a scanning system and omits the control of emission from the electron gun 110 and the focusing of the condenser lens 111 and the objective lens 114. As described above, the functions of the SEM are interrupted when exposure is made by the electron beam, so that the column control unit 103 for making electron beam exposure and SEM operation and the image recording unit 107 necessary for SEM operation are made independent as shown in the figure.
The image recording unit 107 includes an A/D converter 307 for converting the output of the secondary electron detector 115 to a digital signal, an image memory 304 for temporarily storing the output of the A/D converter 307, and an image conversion unit 303 for converting a predetermined amount, at a time, of image data stored in the image memory 304 to an image signal such as an RF signal or video signal.
The column control unit 103 includes a data generator 306 for receiving a control signal from the control computer 101 via a data bus 305, a D/A converter for converting the output of the data generator 306 to an analog signal, and a deflection amplifier 309 for amplifying the output of the D/A converter 308 to generate a driving signal for the deflection coil 113. An A/D converter 310 is provided to check the operation of the deflection amplifier 309 and convert the output of the deflection amplifier 309 to a digital signal and transfer it to the data bus 305. The control computer 101 checks the output of the A/D converter 310 via the data bus 305 thus the output of the deflection amplifier 309 to see if the output corresponds to the digital value set in the data generator 306, and the output level is appropriate and linear.
Such checking of the deflection amplifier 309 as described above is made at every predetermined exposure numbers or times. That is, the conventional system is made to check whether the output of the A/D converter 310 corresponds to the value set in the data generator 306. The operation check is made by changing the set value of the data generator 306, so that it takes time to check the next set value after the conversion by the A/D converter 310.
In addition, the column control unit 103 and the image recording unit 107 are made separately, so that it is necessary to provide A/D converters separately, making the system complicated.